The glycosaminoglycans such as heparin, heparan sulfate, dermatan sulfate, chondroitin sulfate and hyaluronic acid are biopolymers that are industrially extracted from various animal organs.
In particular, heparin, mainly obtained by extraction from pig intestinal mucosa or from bovine lung, is a polydispersed copolymer with a molecular weight distribution from approximately 3,000 to approximately 30,000 D consisting of a mixture of chains basically consisting of a uronic acid (glucuronic acid or iduronic acid) and of an amino sugar (glucosamine) linked by α-1→4 or β-1→4 bonds. In heparin, the uronic unit can be O-sulfated in position 2 and the glucosamine unit is N-acetylated or N-sulfated, 6-O-sulfated, and 3-O-sulfated in approximately 0.5% of the glucosamine units present.
The properties and natural biosynthesis of heparin in mammals have been described by Lindahl et al., 1986 in Lane, D. and Lindahl, U. (Editors) “Heparin. Chemical and Biological Properties; Clinical Applications”, Edward Arnold, London, Pages 159-190, by Lindahl, U, Feingold D. S. and Rodén L, 1986 TIBS, 11, 221-225 and by Conrad H. E. “Heparin Binding Proteins”, Chapter 2: Structure of Heparinoids, Academic Press, 1998. The biosynthesis of heparin occurs starting from its precursor N-acetyl-heparosan consisting of a mixture of chains consisting of the repetitive disaccharide unit glucuronyl-β-1→4-N-acetylglucosamine. Said precursor undergoes enzymatic modifications which partially hydrolyze the N-acetyl group, replace it with an SO3− group, epimerize the carboxyl in position 5 of a part of the glucuronic units, convert them into iduronic units and introduce O-sulfate groups to get a product which, once extracted industrially, has approximately a double number of sulfate groups in respect of the carboxy groups per disaccharide unit. These enzymatic modifications lead, among other things, to the formation of the pentasaccharidic antithrombin III (ATIII) binding region, called active pentasaccharide, which is the structure necessary for the high affinity bond of heparin to ATIII and fundamental for the anticoagulant and antithrombotic activity of heparin. This pentasaccharide, present inside only some of the chains which form heparin, contains a sulfated glucosamine unit in position 3 and a glucuronic acid spaced out between disaccharides containing iduronic acids.
In nature, the formation of the active pentasaccharide is made possible by the epimerization reaction of the carboxyl of a part of the glucuronic units into iduronic units provided by the D-glucuronyl C5-epimerase (C5-epimerization) and by a suitable sulfation which also leads to the introduction of a sulfate group onto the hydroxyl in position 3 of the glucosamine. More particularly, in nature the formation of the active pentasaccharide is made possible by the fact that the C5-epimerization occurs in clusters, i.e. on portions of chains, and extensively, which results in a product that contains more iduronic units than glucuronic ones. Commercial heparin, in fact, contains approximately 70% iduronic units and 30% glucuronic units.